Ongoing technology and standardization developments make the use of large antenna arrays at cellular base stations and other wireless access points a viable option to boost the air interface capacities and maximum data rates of wireless communication networks. Consider a base station or an access point equipped with a large number of antennas. The node can simultaneously schedule multiple wireless devices in the same time/frequency band, using simple linear processing such as maximum-ratio transmission or zero-forcing in the downlink and maximum-ratio combining or zero-forcing in the uplink. Current literature often refers to these multi-antenna arrangements as multiple-input-multiple-output, or MIMO.
MIMO can therefore be understood as an advanced antenna technique that improves spectral efficiency and boosts overall system capacity. MIMO can be used for achieving diversity gain, spatial multiplexing gain and beamforming gain. The MIMO technique uses a commonly known notation (M×N) to represent a MIMO configuration in terms of the number of transmit (M) and receive antennas (N) involved. The common MIMO configurations used for various technologies are: (2×1), (1×2), (2×2), (4×2), (8×2) and (2×4), (4×4), (8×4). In addition, the Third Generation Partnership Project, 3GPP, has discussed extending the number of antennas at the base station up to 16/32/64.
It is well known that MIMO systems can significantly increase the data carrying capacity of wireless systems. For these reasons, MIMO is an integral part of the third and fourth generation wireless systems. In addition, massive MIMO systems are currently under investigation for fifth generation systems. MIMO systems may also include very large MIMO or VL-MIMO. VL-MIMO systems are also sometimes referred to as “full dimension” or FD systems.
In 3GPP Long Term Evolution, LTE, antenna mapping can be understood as a mapping from the output of the data modulation circuitry to the different antennas ports. The input to the antenna mapping thus includes the modulation symbols, such as QPSK, 16QAM, 64QAM, 256QAM symbols, corresponding to the one or two transport blocks. To be more specific, there is one transport block per Transmission Time Interval or TTI, except for spatial multiplexing, in which case there may be two transport blocks per TTI. The output of the antenna mapping is a set of symbols for each antenna port. The symbols of each antenna port are subsequently applied to the orthogonal frequency-division multiplexing, OFDM, modulator. That is, the symbols are mapped to the basic OFDM time-frequency grid corresponding to that antenna port.
Beamforming in the downlink represents an aspect of MIMO and preceding includes multiplying the signal with different beamforming weights for each antenna port prior to transmission. Base stations use beamforming to focus transmitted energy towards desired users—i.e., towards the wireless devices, or user equipments, UEs, being served at any given time. Focusing the radiated energy boosts coverage and raises the maximum data rates achievable on the downlink under real-world channel conditions. Accurate Channel State Information or CSI is a requisite for effective beamforming and acquiring accurate CSI in a scalable fashion for MIMO systems is non-trivial. In conventional systems, radio network nodes transmit per-antenna pilot signals, and UEs estimate downlink channel gain based on measurements of the pilot signals.
Regarding downlink data transfer in LTE, the UE computes the channel estimates from the pilot or reference signals and then computes the parameters needed for CSI reporting. The CSI report includes, for example, a Channel Quality Indicator or CQI, a Precoding Matrix Index PMI or PMI, and/or Rank Information, denoted as RI. The CSI report is sent to the eNodeB via a feedback channel, which is either a Physical Uplink Control Channel, PUCCH, for periodic CSI reporting, or a Physical Uplink Shared Channel, PUSCH, for aperiodic reporting. The eNodeB scheduler uses this information in choosing the parameters for scheduling of the UE. The eNodeB sends the scheduling parameters to the UE in the Physical Downlink Control Channel or PDCCH. The scheduling information includes a number of MIMO layers scheduled, transport block sizes, modulation for each codeword, parameters related to HARQ, and sub band locations. Subsequently, the actual data transfer takes place from the eNodeB to the UE.
Active-array-Antenna Systems or AAS integrate radio frequency power amplifiers and transceivers with an array of antennas elements and offer several benefits compared to traditional deployments with passive antennas connected to transceivers through feeder cables. Passive antennas array systems boost baseband signals, but are connected to the antennas by longer feedback cables. AAS reduces cable losses, improves performance, reduces energy consumption, requires less space for implementation, and simplifies installation.
There are many applications of AAS, such as cell-specific beamforming, user-specific beamforming, vertical sectorization, massive MIMO, vertical beamforming, and so on. AAS may also enable further-advanced antenna concepts, such as deploying a large number of MIMO antenna elements at the eNodeB. For these reasons, 3GPP started a study item investigating the feasibility of increasing the number of transmit antennas to 16/32/64 for various purposes and also extending the CSI feedback to support two-dimensional antenna arrays.